Fenceposting with atTime?

[mookerji]

This is mildly related to #44. I was writing some of my own tests my use of TimeSeries when I noticed that I was missing some events when using atTime and bisect. Here's an example:

      let eventSource = new UnboundedIn();
      let collection = new Collection();
      let timeseries = new TimeSeries({name: "test",
                                       collection: collection});
      let pipeline = new Pipeline()
          .from(eventSource)
          .to(EventOut,
              event =>
              { collection = collection.addEvent(event);
                timeseries = new TimeSeries({name: "test",
                                             collection: collection});});
      assert(collection.size() == 0);
      assert(timeseries.size() == 0);
      eventSource.addEvent(new Event(time1, {value: 2}));
      assert(collection.size() == 1);
      assert(timeseries.size() == 1);
      console.log("timeseries" + timeseries.toString()); 
        // => {"name":"test","utc":true,"columns":["time","value"],"points":[[1465084800000,2]]}
      console.log("time1 index " + timeseries.bisect(time1)); 
        // => 0
      console.log("index 0 " + timeseries.at(0)); 
        // => {"time":1465084800000,"data":{"value":2}}
      console.log("using timeAt " + timeseries.atTime(time1)); 
        // => undefined :(

I think this is because at https://github.com/esnet/pond/blob/master/src/pond/lib/collection.js#L192, instead of if (pos && ....) you actually want if (pos >= 0 && ...).